Review Article Civil Eng Res J Volume 2 Issue 3 - September 2017 Copyright © All rights are reserved by A Patel DOI: 10.19080/CERJ.2017.02.555589 Classification of Weathering in Rocks and Its Engineering Implications
A Patel* Department of Civil Engineering, Visvesvaraya National Institute of Technology, India Submission: September 06, 2017; Published: September 22, 2017 *Corresponding author: A Patel, Department of Civil Engineering, Visvesvaraya National Institute of Technology, Nagpur-440010, India, Email:
Abstract
description on several factors causing weathering in rocks followed by the weathering mechanism and the description of weathering in rocks. A reviewA critical of existing review codes on theand description guidelines onand weathering classification and of its weathering implications in rocks to describe has been the presented engineering in thisproperties paper. Thisof rocks paper has starts been with presented. a brief From the point of view that weathering process modify the mass engineering properties of rocks which are quite unpredictable, this type of review work becomes necessary. The present paper forms a useful source of information for geologists and geotechnical engineers working on weathered rocks.
Keyword:
Rock; Weathering; Classification; Guidelines; Engineering properties
Introduction after the construction has been over. Efforts have been made by the researchers, engineers and geologists to classify weathered differently by different authors [1-4]. However, weathering Weathering of rocks is a purely subjective term and is defined rocks and its effects on various engineering properties of rock in general is the process which brings about several changes mass. Moreover, codes and guidelines have been formulated to the properties of rock mass due to its exposure to changing by the individuals (Table 1) and the international societies environmental (physical, chemical and biological) conditions and like International Society for Rock Mechanics [17] and the hence from the engineering point of view, grading of weathering Engineering Group of Geological Society Working Party. In this becomes very important. The alteration in the engineering paper, a review on various aspects of these guidelines and its properties of rocks due to weathering may occur at the engineering implications has been highlighted. material scale (rock strength, swelling potential, slaking index, compressibility, consolidation characteristics etc.) and/or mass Weathering of Rocks scale (discontinuities, fracture, joint, settlement, permeability Weathering in rocks is a very complex phenomenon that can etc.), as reported in the literature [4-12]. Moreover, behavior of be presented in a matrix form in (Figure 1). It is not only the the weathered rocks depends upon the environmental conditions, environmental conditions and the rock properties on which the which it is further exposed to. For example, weathered rocks in rate of weathering will depend but also the combination of these dry condition may swell or start disintegrating, when coming in factors, duration of exposure and the types of environmental contact with water [13]. Similarly, different applications place cycles (like wetting-drying, freeze-thaw etc.) that matters the emphasis on different attributes of the weathered rocks [14]. most. Moreover, it is the distribution and concentration of the When constructing a building on weathered rocks, the engineer weather-resistant (e.g., quartz, feldspar etc.) and weather prone will be interested more on its bearing capacity; whereas in case minerals (e.g., biotitic, muscovite, hornblende etc.), that will of slopes and embankments he is more concerned with changes affect the grade of weathering [4,18-20]. in permeability as a result of weathering [15]. However, grading In spite of these limitations, some of the researchers [16,20- 23] have tried to correlate the rate of weathering through weathered rocks is a very difficult task and as a result, the rock is quite unpredictable and can’t follow any calculation various laboratory tests including X-ray diffraction (XRD) modification in various engineering properties of a weathered technique [4,16]. In addition, engineering may expose the rock analysis; study of minerals using Scanning electron microscope mass to a new environment and weathering may continue even
(SEM); measurement of specific surface area (SSA), total cation Civil Eng Res J 2(3): CERJ.MS.ID.555589 (2017) 0053 Civil Engineering Research Journal exchange capacity (TCEC), type of exchangeable cations and the such as basalt but is less on white rocks like chalk [4]. On the pH value. These relationships are very helpful for understanding the weathering mechanism in different types of rocks. For be more on chalks than basalt. other hand, weathering due to the effects of chemical fluids will example, weathering due to temperature is more on dark rocks
Figure 1: A matrix of various parameters causing weathering in rocks.
Rocks of sedimentary and metamorphic origin (such as clay through oxidation and reduction, hydration and dehydration, shale’s, poorly to moderately cemented sandstones, marl, and ion exchange and leaching process [23]. Higgs [24] has reported anhydrite) are very moisture-sensitive and hence get weathered due to reaction with water [14]. Basic and ultra basic igneous resistant as compared to the coarse-grained types. Moreover, that igneous rocks with fine-grain sizes are more weather rocks, metamorphic rocks, limestone, sandstone and mudshale, the deterioration in some of the rocks like basalt increase which contain plenty of silicate minerals, easily convert into clay minerals such as montmorillonite, illite and kaolinite itself are product of weathering of the fresh rocks [13,24,25,]. significantly due to the presence of secondary minerals which Table 1: Some classification systems for weathered rocks. References Major Descriptive Terms Origin/Applicability Joints strain, Rock and soil material strain, Feldspar decomposition, Strength of NX core samples, Weathering of granites in the Snowy Moye [26] Disintegration in water, Biotitic decomposition and Mountains, Australia presence or absence of original texture Ruxton & Berry [27] Concept of rock : soil ratio Weathered granite in Hongkong Little [28] Concept of rock : soil ratio Weathered granite Discoloration, decompositions, disintegration and Dearman [6], Anon [29]: BS 5930 Mass weathering grade freshness condition-terms combined with descriptive terms Wide range of rock types in varying structural Stapledon [30] for rock material strength conditions, Australia granitic residual soil and sedimentary Dames & Moore [31] residual soil, Singapore MaterialModification fabric (texture), to BS discoloration, 5930 Metamorphic rock (Clastic Metasediment) in Komoo & Mogana [32] discontinuity surface, Peninsular Malaysia Uniformity or regularity and style of weathering Price [4] (uniform, complex, core stone and solution Mass weathering for engineering purposes weathered) Changes in color; Changes in fracture state; Engineering Group of Geological Society Reduction in strength; and Presence, character and Engineering purpose Working Party [38] extent of weathering products
How to cite this article: A Patel.Classification of Weathering in Rocks and Its Engineering Implications. Civil Eng Res J. 2017; 2(3): 555589. DOI: 0054 10.19080/CERJ.2017.02.555589 Civil Engineering Research Journal
Cluster analysis combining several tests (water absorption, magnesium sulphate, petro graphic number, rock expansion, High-Low T-cycles, Freeze- Hudec [16] Engineering purpose thaw cycles, wetting-drying cycles, absorption SSD, Impact strength, Los Angle abrasion, bulk density) with similar results into groups.
disintegration of dolomitic components, Lempe et al. [12] Decalcification and associated browning, carbonate northern Alpine foreland (Bavaria/ Pleistocene glaciofluvialGermany) gravel rich in of volume/mass acidification and an increase of the clay content, loss Description of weathering in rocks of environmental and rock conditions have been made. Due to the effect of weathering, there occurs an overall With these limitations, different classifications for different sets change in the engineering properties of rock mass. Hence, from However, it is observed that the descriptive terms used for the engineering point of view, it becomes very important to classifying weathered rocks are purely subjective. For example, classify the weathering state of a rock mass. Various researchers, the color seen by one person will depend on the type of light engineers and geologists [4,6,12,16,26-32] have tried to do source, the background, the size of the object and the colors that so in the past (Table 1). It might be easy to understand the have been seen immediately before [14]. Moreover, many times mechanism or process of weathering in rocks through various it has been seen that there is a mismatch of these descriptive terms (e.g. colors, decomposition, disintegration, discontinuities, earlier, weathering is a purely subject term and there is no fracture spacing etc.) in the core logs and core photographs due laboratory tests, but very difficult to quantify it. As mentioned such calculation technique or laboratory tests available, that to their subjective in nature. The color of the rock samples just can quantify the amount of weathering in rock mass. Moreover, or nearly after drilling will vary from that taken after few hours depending upon the types of engineering works, the rock mass depending upon the weather condition on site as shown in will be further exposed to weathering that can’t be predicted. (Figure 2).
Figure 2: Change of color in rock core samples with time.
The concept of soil rock mass ratio [27,28] seems to be the changes in index properties of the weathered rock mass. a better option to quantify the grade of weathering. Some of A very few attempts [11] have been made to relate weathered the researchers [33] have tried to gauge rate of weathering in rocks with its engineering properties. It can be mentioned here of 14C methods. However, the soil mass ratio will again depends (SPT) blow count and the shear and compression wave velocities relation with specific secondary mineral formation with the help that some of the field test results like standard penetration test upon the friability or slaking nature of the rock mass and the can precisely represent the weathering state in different rock environmental conditions to which it is further exposed to. mass. Donald et al. [19] suggested that the percentage of core recovery is an indication of soundness and degree of weathering
Interestingly, most of the classification systems are based upon
How to cite this article: A Patel.Classification of Weathering in Rocks and Its Engineering Implications. Civil Eng Res J. 2017; 2(3): 555589. DOI: 0055 10.19080/CERJ.2017.02.555589 Civil Engineering Research Journal of rock. Hence it is very important that relative measurements when rock is exposed [37]. The above mentioned are a few of the examples, that highlights the responsibilities of the engineers compression or shear wave velocities to laboratory values. and designers, when dealing with weathered rocks. of rock quality is made by comparing ratios of field values of tests like SPT, CPT or Pressure meter tests. With a difference from Concluding Remarks Otherwise, it better to rate the weathering directly from the field Weathering in rocks is a subjective term and hence it is description of mass weathering for engineering purposes, the the other classification systems, Price [4] pointed out that in the uniformity or regularity and style of weathering is a key issue. can be tackled easily with proper interpretation of the data difficult to quantify it. However, problems in weathered rocks Keeping this in view, he suggested to classify mass weathering available, both at the material and mass level. The descriptive as uniform(ly) weathered; complex(ly) weathered; core stone weathered or solution weathered. mostly the index properties of rocks and are subjective in nature. terms commonly used for classification of weathered rocks are Attempts should be made to correlate the weathering state of Due to the fact that a large range of rock types and masses rocks to its engineering properties. and weathering processes exists, there are many possibilities References 1. Reiche P (1950) A survey of weathering processes and products. for future classifications of particular rock masses weathered of weathering in rocks for engineering purposes should lie in University of New Mexico Publications in Geology, 3, University of New in particular environments. It is suggested that classification Mexico Press, New Mexico, United States. to establishing the boundary between that part of the weathered 2. Ollier C (1991) Ancient Landforms. 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How to cite this article: A Patel.Classification of Weathering in Rocks and Its Engineering Implications. Civil Eng Res J. 2017; 2(3): 555589. DOI: 0056 10.19080/CERJ.2017.02.555589 Civil Engineering Research Journal
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How to cite this article: A Patel.Classification of Weathering in Rocks and Its Engineering Implications. Civil Eng Res J. 2017; 2(3): 555589. DOI: 0057 10.19080/CERJ.2017.02.555589